1. Field of the Invention
The present invention relates to a semiconductor device with a resistor formed from polycrystalline silicon film and having high resistivity with a small area.
2. Description of the Related Art
In an analog semiconductor device, a resistor is widely used as an important device. Constructing a resistor from polycrystalline silicon film, the resistor is required to have the minimum width and thickness of the polycrystalline silicon film should be thin so as to obtain a desired resistance with a small area. The width of the resistor is, however, determined by accuracy of process. In addition, when the thickness of the film is thin, there arises a problem in that over-etching of the polycrystalline silicon film and penetration to the underlying structure occur in an etching process for taking an opening for contact in an electrode lead-out region.
As a countermeasure for the problem described above, as shown in FIG. 15, a field insulating film 201 is formed on a semiconductor substrate 101, and a resistor portion formed thereon is constituted of a second polycrystalline silicon film 303 whose film thickness is thin. In order to prevent the over-etching and penetration at a portion in which an opening for contact is made, there is a method of providing a first polycrystalline silicon film 302 whose thickness is thick and an oxide film 206 formed thereon under the second polycrystalline silicon film 303. (For example, see JP 06-69207 A)
There is another conventional method by which a region of a polycrystalline silicon film which becomes a resistor is subjected to thermal oxidation, which makes the film thickness thin and increases the resistivity thereof, while the thickness of the polycrystalline silicon film at the electrode lead-out region is kept thick, to thereby prevent the over-etching and penetration upon making an opening for contact. (For example, see JP 2004-140062 A)
In the invention described in JP 06-69207 A, electrical coupling between a second polycrystalline silicon film and a first polycrystalline silicon film is performed through contact of a side surface of the first polycrystalline silicon film with the second polycrystalline silicon film. In this case, there arises a problem in that preferable electrical coupling cannot be obtained without sufficient removal of polymer or reaction product formed on a side wall of the first polycrystalline silicon film at the time of etching of the first polycrystalline silicon film, and of natural oxide film formed on the first polycrystalline silicon film before the deposition of the second polycrystalline silicon film.
Further, in the invention described in JP 2004-140062 A, thermal oxidation is employed to make the polycrystalline silicon film, which becomes a resistor, into a thin film. There arises a problem in that, in a case of making a transistor on the same semiconductor substrate as that for the resistor with the use of a polycrystalline silicon film obtained by adding a p-type impurity such as boron for a gate electrode, the impurity introduced in the polycrystalline silicon film is rediffused into a channel region of the transistor, and causes a threshold voltage shift of the transistor and increases instability of the transistor, in thermal oxidation performed after formation of the gate electrode.